AN ION-CHANNELING STUDY OF IRRADIATION-INDUCED AMORPHIZATION OF YBA2CU3O7-X

We previously employed Brillouin scattering to demonstrate that the irradiation-induced crystalline-to-amorphous (c-a) transformation in several intermetallic compounds was preceded by a large (> 50%) decrease in shear elastic constant, indicative of an elastic instability. In situ electron diffraction revealed that the elastic softening was caused by static atom displacements. Here we report the results of an ion-channeling study designed to examine whether parallel changes occur during ion irradiation of single crystals of YBa2Cu3O7-x. The (static plus thermal) atomic displacements introduced during 1.5 MeV He-4 irradiation at a temperature of 100 K were determined from [001] axial channeling scans taken after several irradiation increments between 0 and 0.04 dpa. It was possible to obtain reliable channeling information at quite low doses, as low as 0.0004 dpa, because of the excellent minimum yields (chi-min < 3%) obtained from the high-quality single crystals. In agreement with the previous observations in intermetallic compounds, large atomic displacements, implying a drop in elastic modulus of greater-than-or-similar-to 90%, were found prior to the onset of amorphization. However, chi-min was found to be directly proportional to dose over a very broad range. The observed linear dependence on dose indicates that the irradiation-induced disorder develops continuously, and at the same rate from the very onset of the irradiation to nearly complete amorphization.